Re: [w3c/browser-payment-api] Constructor should not include "total" in list of items (#18)

If a merchant doesn't know the final cost of the service they're providing - for example, a ride sharing company may use the sub-totals to indicate the per-mile or per-minute cost of the service, but the total will be the amount to be initially pre-authorized (in such scenarios you also benefit from being able to mark sub-total or total items as pending rather than actual, but that's a matter for another issue). Or, as @halindrome says, where the sub-total list isn't comprehensive.

Putting that aside, if you don't have an explicit total then you're relying on the merchant not to make a mistake in the construction of their sub-totals (or the user-agent to not make a mistake summing them. Neither of these are a given). Having an implicit rather than explicit total introduces several potential failure points for no real benefit. It is much better for the merchant to be explicit about the amount they wish to charge. 



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